Nano-powder-based coating and ink compositions

ABSTRACT

Nano-powder-based coating and ink compositions, methods for producing and using these compositions, and articles prepared from these compositions are described.

This application is a continuation-in-part of the following commonlyowned, pending applications, each of which is hereby incorporated byreference in its entirety:

(1) PCT/IL03/00502 entitled “A Method for the Production of Conductiveand Transparent Nano-Coatings and Nano-Inks and Nano-Powder Coatings andInks Produced Thereby,” filed Jun. 12, 2003 designating the UnitedStates, which derives priority from U.S. Provisional Application No.60/387,919 filed Jun. 13, 2002;

(2) PCT/IL2003/000554 entitled “Low Sintering Temperatures ConductiveNano-Inks and a Method for Producing the Same,” filed Jul. 3, 2003designating the United States, which derives priority from U.S.Provisional Application No. 60/393,123 filed Jul. 3, 2002; and

(3) U.S. Provisional Application No. 60/609,751 entitled “LowTemperature Sintering Process for Preparing Conductive Printed Patternson Substrates, and Articles Based Thereon,” filed Sep. 14, 2004.

FIELD OF THE INVENTION

The present invention generally relates to the production and use ofnano-powder-based coating and ink compositions.

BACKGROUND

Nano particles, especially metal nano particles have very specialproperties which are directly related to their dimensions and to thefact that a large ratio of the atoms in the particle are in the surfaceof the particle or at particle and grain boundaries. These propertiesinclude optical properties, sintering and diffusion properties,electrical properties like capacitance, impedance and resistance,catalytic activity and many others.

These improved properties have a range of uses and applications; e.g.catalysts for chemical reactions, electrodes, fuel cells, medicaldevices, water cleaning technologies, electronic devices, coatings, andmore.

U.S. Pat. No. 5,476,535 to the applicant present a method for theproduction of nano-powders, especially of silver. This processcomprising the steps of (a) forming an aluminum-silver alloy of aspecific blend composition; (b) leaching the aluminum ingredient by aseries of consequent leaching steps wherein a fresh leaching agent isreacting the treated solid material, providing a gradually porous andhomogeneous silver alloy. Ultrasonic oscillations are applied in step(c), disintegrating the agglomerate and enhancing the penetration of theleaching agent into the ever growing porous of the alloy by theapplication of a plurality of ultrasonic oscillations. The leachingagent is leaving silver agglomerate in step (d), and then theagglomerate is washed and dried in a final step.

According to U.S. Pat. No. 6,012,658 to the applicant et al., the verysame process was used as is to form metal flakes. Thus, the followingtwo main steps were introduced: comminuting the alloys obtained by theaforementioned U.S. Pat. No. 5,476,535 into defined particles, and thenfaltering the obtained particles into strip-like highly porous alloys ofpredetermined characteristics.

SUMMARY

In one aspect, there is described a method for coating a substrate byconductive and/or transparent nano-powders in either ordered or randompattern, wherein said pattern is obtained either spontaneously or as aresult of a printing technique. It is well in the scope of the presentinvention to use nano metal additives to produce atransparent-conductive coating such in the case of nano titanium dioxideadditives that was proved useful for the production of coatingstransparent to visible light and opaque to UV wave lengths; in the caseof nano silica additives, that was found useful for the production oftransparent coatings with special resistance performance; and in thecase of nano pigments, that proved useful for the production oftransparent colored coatings.

In another aspect, there is described a method for the production ofconductive and transparent coatings comprising metal nano-powders. Saidmethod comprising the steps of (i) admixing metal nano powder in asolvent with at least one ingredient of the group: binder, surfactant,additive, polymer, buffer, dispersant and/or coupling agent in themanner a homogenized solution is obtained; (ii) applying the homogenizedmixture obtained above on a surface to be coated by various manners:screen printing, spreading, spin coating, dipping etc.; (iii)evaporating the solvent from said homogenized mixture; and (iv)sintering the coated layer so a conductive and transparent coating isobtained on top of said surface.

The metal nano powders may be used alone or in combination withconductivity improving additives selected from at least one of thegroup: metal colloids and/or metal reducible salt and/or organic metalcomplexes and/or organo metal compounds which decompose to formconductive materials. Preferably, the concentration of the metal nanopowder in the admixed solution is between 1% (wt) to 50% (wt) and moreparticularly, in the range of 2% (wt) to 10% (wt).

It is further in the scope of the present invention wherein the admixedsolution comprises organic solvent or a mixture of organic solvents.Those organic solvents are characterized by an evaporation rate higherthan the evaporation rate of water at ambient conditions. Theconcentration of the organic solvent or the mixture of organic solventsin the admixed solution is between 20% (wt) to 85% (wt), morespecifically, in the range 40% (wt) to 80% (wt). It is acknowledged inthis respect that the solvents can be selected from at least one of thegroup of petroleum ether, hexanes, heptanes, toluene, benzene,dichloroethane, trichloroethylene, chloroform, dichloromethane,nitromethane, dibromomethane, cyclopentanone, cyclohexanone, UV andthermally curable monomers (e.g., acrylates), or any mixture thereof.

It is further in the scope of the present invention wherein theconcentration of the aforementioned binder in the admixed solution isbetween 0% (wt) to 3% (wt). Preferably (but not limited to), the binderis selected from ethyl cellulose and/or modified urea.

In yet another aspect, there is described a method for the production ofinks or solutions comprising metal nano-powders for the production oftransparent and conductive coatings. This method is in principle similarto the one defined above, and comprises the following steps: (i)admixing metal nano-powder in a solvent with at least one ingredient ofthe group: binder, surfactant, additive, polymer, buffer, dispersantand/or coupling agent in the manner a homogenized solution is obtained;(ii) admixing said homogenized mixture with water or water misciblesolvent or mixture of water miscible solvents in the manner a W/O typeemulsion is obtained; (iii) applying the emulsion obtained above on saidsurface to be coated, e.g., by spreading, spin coating, dipping etc.;(iv) evaporating the solvent from said homogenized mixture in the mannerthat a self-assembled network-like pattern is developed in situ; andlastly; (v) sintering the network-like pattern so a conductive andtransparent coating is obtained.

It is in the scope of the present invention wherein the concentration ofthe aforementioned surfactant or surfactants mixture is between 0% (wt)to 4% (wt) and/or wherein the concentration of the surfactant orsurfactants mixture in the dispersed emulsion is between 0% (wt) to 4%(wt). Preferably, a W/O emulsion is obtained by said methods.

It is further in the scope of the present invention wherein theconcentration of the water miscible solvent or mixture of water misciblesolvents in the dispersed emulsion is between 5% (wt) to 70% (wt).Preferably (but not limited to), the surfactant or surfactants mixturedefined above comprises at least one of the group of non-ionic and ioniccompounds, selected from SPAN-20, SPAN-80, glyceryl monooleate, sodiumdodecyl sulfate, or any combination thereof. Moreover, it is also in thescope of the present invention wherein the concentration of the watermiscible solvent or mixture of water miscible solvents in the dispersedemulsion is between 15% (wt) to 55% (wt).

It is further in the scope of the present invention wherein theaforementioned water miscible solvent or solvent mixture is selectedfrom (but not limited to) at least one of the group of water, methanol,ethanol, ethylene glycol, glycerol, dimethylformamid, dimethylacetamid,acetonitrile, dimethylsulfoxide, N-methylpyrrolidone or any mixturethereof.

There is also described a method, as defined in any of the above,wherein the surface to be coated is selected from paper, metal,ceramics, glass, either flexible or relatively non-flexible polymericfilms or sheets or any combination thereof. More specifically, thepolymeric film comprises at least one of the group of polyesters,polyamides, polyimides (e.g., Kapton), polycarbonates, polyethylene,polyethylene products, polypropylene, acrylate-containing products,polymethyl methacrylates (PMMA), their copolymers or any combinationthereof, or any other transparent or printable substrate. Additionallyor alternatively, the method defined above comprises another step oftreating the surface to be coated by a means of corona treatment and/ora primer.

It is further in the scope of the present invention wherein theaforementioned primer is selected (but not limited to) from at least oneof the group of 3-aminopropyl triethoxy silane, phenyl trimethoxysilane,glycidyl trimethoxysilane, commercially available Tween products,Tween-80, neoalkoxy tri(dioctylpropylphosphato) titanate or anycombination thereof.

It is further in the scope of the present invention wherein theaforementioned nano-powder comprises metal or a mixture of metals(including metal alloys) selected from (but not limited to) silver,gold, platinum, palladium, nickel, cobalt, copper or any combinationthereof.

It is further in the scope of the present invention wherein thespreading of the homogenized mixture on a surface to be coated isprovided by a means selected from simple spreading; bar spreading;immersing; spin coating; doping; dipping or any other suitabletechnique. Moreover, according to one embodiment of the presentinvention, the step of coating layer or layers provided by the spreadingof the homogenized mixture on a surface to be coated is provided for awet thickness of 1 to 200 microns, e.g., 5 to 200 microns.

It is further in the scope of the present invention wherein thesintering of the evaporated homogenized solution is provided in thetemperature range of 50° C. to 300° C. for 0.5 to 2 hours.Alternatively, the sintering of the network-like pattern is provided inthe temperature range of 50° C. to 150° C. for 2 to 30 minutes.

It another object of the present invention to provide a cost-effectiveand novel conductive and transparent coating layer comprising metalnano-powders, as described above. Additionally or alternatively, aconductive and transparent ink or coating layer or layers is providedthat is characterized by self-assembled network pattern.

It is also in the scope of the present invention wherein theaforementioned conductive and transparent coating layer is characterizedby light transparencies in the range of 30% to 90% at 400 nm to 700 nmwavelength; resistances in the range of 0.1 Ω/square to 10 kΩ/square andby haze value in the range of 0.5% to 10%; and wherein the conductiveand transparent ink layer is characterized by light transparencies inthe range of 10% to 90% at 400 nm to 700 nm wavelength; resistances inthe range of 0.1 Ω/square to 1,000 Ω/square, and by a haze value in therange of 0.1% to 5%. Also described are conductive inks (e.g.,nano-powders characterized by resistances between 0.005 Ω/square to 5kΩ/square) comprising metal nano-powders.

It is further in the scope of the present invention wherein theconductive and transparent coating layer defined above is characterizedby either ordered or random patterns, wherein said patterns are providedby printing, ink-jet printing, ink-jet disposition, self-assembling orself organizing or any other suitable technique. Said conductive andtransparent coating layer or the multiple layer array may further beprovided with a protective layer, anti-scratch layer, a layer to enhanceconductivity, a layer to enhance adhesion to a surface to be coated orany combination thereof. Moreover, the obtained conductive andtransparent coating layer or the aforementioned multiple layer array maybe especially adapted to be used in at least one of the group ofscreens, displays, electrodes, PCBs, ink-jet products, ink-jetdisposition products, smart cards, RFID, antenna, thin-film transistors,LCDs or any combination thereof.

Also described are methods for producing conductive inks comprisingmetal nano-powders where the method comprises inter alia the followingfour steps: (i) admixing metal nano powder in a solvent with at leastone ingredient of the group selected from: binder, surfactant, additive,polymer, buffer, dispersant and/or coupling agent in the manner ahomogenized solution is obtained; (ii) applying the homogenized mixtureobtained above on a surface to be coated; (iii) evaporating the solventfrom said homogenized mixture; and (iv) sintering the coated layer at atemperature range of 50° C. to 350° C., providing a conductive ink ontop of said surface characterized by resistances between 0.005 Ω/squareto 5 kΩ/square. Said sintering is preferably provided at ambientpressure (e.g., about atmospheric pressure).

Also described is a low temperature (e.g., less than 100° C., preferablyless than 70° C., and even more preferably at room temperature)sintering process for sintering compositions comprising nano metalparticles deposited (e.g., printed) on a substrate. Resistivitiesfollowing sintering are below about 300 μΩcm, preferably below about 100μΩcm, and more preferably below about 30 μΩcm. Resistivities may befurther reduced using techniques such as electroplating followingdeposition.

The sintering process is conducted in the presence of a chemical thatinduces the sintering process. Examples of suitable chemicals includeformic acid, acetic acid, and formaldehyde. The chemical may be in theform of a vapor or a liquid to which the deposited particles areexposed. Alternatively, it may be incorporated into the compositioncomprising the nano metal particles prior to deposition, or may bedeposited on the nano metal particles after depositing the particles onthe substrate.

A wide variety of substrates can be used, including both flexible andrigid substrates, polymer films (e.g., polyethylene terephthalate andpolyolefins such as biaxially oriented polypropylene), plastics, paper(including photographic paper), textiles, printed circuit boards, epoxyresins, and the like. The process is particularly useful for substratessuch as paper or polymer films having Tg's or melting points under about150° C., which cannot withstand high sintering temperatures (e.g., 300°C. or higher).

Suitable nano metal particles include silver, silver-copper alloys,silver palladium or other silver alloys or metals or metals alloysproduced by the process Metallurgic Chemical Process (MCP) described inU.S. Pat. No. 5,476,535 (“Method of producing high purity ultra-finemetal powder”) and PCT application WO 2004/000491 A2 (“A Method for theProduction of Highly Pure Metallic Nano-Powders and Nano-PowdersProduced Thereof”). The powders produced hereby have a “non uniformspherical” deformed ellipsoidal, worm like shape (see FIGS. 10 and 11)and its chemical composition can include up to 0.4% (by weight) ofaluminum, both of which are unique to this production method.

Particularly useful compositions (inks and dispersions) for depositingthe nano metal particles include ink jet printable compositions in whichthe particles are combined with a liquid carrier as described inProvisional Application No. 60/609,750 entitled “Ink Jet PrintableCompositions” filed Sep. 14, 2004, which is incorporated by reference inits entirety. These inks and dispersions have properties that enabletheir jettability (printing through ink jet heads which possess smallnozzles, usually in the micron range). These properties include thefollowing: low viscosities between 1 and 200 cP (at room temperature orat jetting temperature), surface tension between 20-37 dyne/cm forsolvent-based dispersions and 30-60 dyne/cm for water-based dispersions,metal loadings of nano particles between 1% and 70% (weight by weight),low particle size distribution of the metal nano-particle materialhaving a particle size distribution (PSD) D90 below 150 nm, preferablybelow 80 nm. The compositions have stabilities sufficient to enablejetting with minimum settling, and without clogging the print head orchanging the properties of the compositions. The compositions can beprinted by different technologies, including continuous ink jettechnologies, drop on demand technologies (such as piezo and thermal),and also additional technologies like air brush, flexo, electrostaticdeposition, wax hot melt, etc.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a view taken by a means of a light microscope showingthe self-assembling pattern of an ink disposed on a glass surface asobtained by the method of one embodiment of the present invention.

FIG. 2 presents a view taken by a means of a light microscope showingthe self-assembling pattern of an ink disposed on a glass surface asobtained by the method of another embodiment of the present invention.

FIG. 3 presents a view taken by a means of a light microscope showingthe self-assembling pattern of an ink disposed on a glass surface asobtained by the method of another embodiment of the present invention.

FIG. 4 presents a view taken by a means of a light microscope showingthe self-assembling pattern of an ink disposed on a glass surface asobtained by the method of another embodiment of the present invention.

FIG. 5 presents a view taken by a means of a light microscope showingthe self-assembling pattern of an ink disposed on a polymeric film asobtained by the method of yet another embodiment of the presentinvention.

FIG. 6 presents a view showing the printed pattern of ink disposed on aglass surface obtained by the method of another embodiment of thepresent invention.

FIG. 7 is a graph illustrating the change of relative resistance forsilver nano-powders coated with surfactant (1) and washed fromsurfactant (2).

FIG. 8 is a graph illustrating the relative resistance dependence ontemperature for the different particle sizes of silver powders and bulksilver.

FIG. 9 is a graph illustrating the relative resistance dependence ontemperature for the different particle sizes of copper powders and bulkcopper.

FIGS. 10 and 11 are Scanning Electron Microscopy Photographs ofrepresentative nano metal particles.

FIG. 12 illustrates a typical ink jet printed pattern.

FIGS. 13 and 14 are Optical Microscopy Photographs of representativeprinted patterns prior to sintering.

FIGS. 15 and 16 are Scanning Electron Microscopy Photographs ofrepresentative printed patterns prior to sintering.

FIGS. 17 and 18 are Scanning Electron Microscopy Photographs ofrepresentative printed patterns after heat sintering at 150° C.

FIGS. 19 and 20 are Scanning Electron Microscopy Photographs ofrepresentative printed patterns after chemical sintering at 70° C.

FIGS. 21 and 22 are Scanning Electron Microscopy Photographs of drymetal nano powders exposed to formic acid.

DETAILED DESCRIPTION

In one aspect, a novel method for the production of conductive andtransparent coatings and inks comprising metal nano-powders (i.e.,coatings or inks) is hereby presented. The hereto-defined method takesfull advantage of the fact that nano sized particles and grains havemuch larger surface areas than bulk materials, have special opticalproperties and can be worked to produce a conductive phase. By coating asubstrate with an ink, solution or paste that was previously dispersed,cost-effective nano-conductive materials and/or conductive transparentcoatings are produced. Those coatings and inks are generallycharacterized by (1) light transparencies between 40% to 90% in thevisible range of about 400 nm to 700 nm wavelength; by (2), resistancesbetween 0.1 Ω/square to 9 kΩ/square, and (3), by a relatively low hazevalues, generally ranges from 1% to 10%.

In another aspect, a novel method of low temperature sintering usefulfor the production of conductive coatings and inks comprising metalnano-powders (i.e., coatings or inks) is hereby presented. By coating asubstrate with an ink, solution or paste that was previously dispersed,cost-effective nano-conductive materials and/or conductive transparentcoatings are produced.

As noted above, the nano-powders (e.g., powders characterized by D₅₀<60nm and D₉₀<100 nm) have a much larger surface area than bulk materials,are characterized by special diffusion properties, and can be processedso a continuous conductive phase is produced at relatively lowtemperatures and a lower energy input.

In some embodiments, the term “coating” refers to any conductive andtransparent optical layer produced in the manner of admixing metal nanopowder in a solvent with at least one ingredient of the group: binder,surfactant, additive, polymer, buffer, dispersant and/or coupling agentin the manner a homogenized solution is obtained; spreading thehomogenized mixture obtained above on a surface to be coated;evaporating the solvent from said homogenized mixture; and thensintering the coated layer so a conductive and transparent coating isobtained on top of said surface.

In other embodiments, the term “coating” refers to any conductive layerproduced in the manner of admixing metal nano powder in a solvent withat least one ingredient of the group: binder, surfactant, additive,polymer, buffer, -dispersant and/or coupling agent in the manner ahomogenized solution is obtained; and then sintering at low temperaturesof 50 to 300° C.

In some embodiments, the term “ink” refers to any ink containingnano-powders of metal or metals, especially emulsion based compositionsprovided for coloring materials, or alternatively, to legend ink(marking ink) suitable for printing on printed circuit boards (PCB's).

More specifically, in some embodiments the term “ink” refers to anyconductive and transparent topical pattern produced in the manner ofadmixing metal nano powder in a solvent with at least one ingredient ofthe group: binder, additive, polymer, buffer, dispersant and/or couplingagent in the manner a homogenized solution is obtained; admixing saidhomogenized mixture with water or water miscible solvent or mixture ofwater miscible solvents in the manner a W/O type emulsion is obtained;spreading the homogenized mixture obtained above on said surface to becoated; evaporating the solvent from said homogenized mixture in themanner that a self-assembled network-like pattern is developed in situ;and then sintering the network-like pattern so a conductive andtransparent ink is obtained.

In other embodiments, the term “ink” refers to any conductive topicalpattern produced in the manner of admixing metal nano powder in asolvent with at least one ingredient of the group: binder, additive,polymer, buffer, dispersant and/or coupling agent in the manner ahomogenized solution is obtained; optionally admixing the solution withwater or water-miscible solvent or a mixture of water-miscible solventsin the manner a W/O type emulsion is obtained; spreading or printing thehomogenized mixture obtained above on said surface to be coated;evaporating the solvent from said homogenized mixture in the manner thata self-assembled network-like pattern is developed in situ or a printedpattern or a complete coverage is formed; and then sintering thenetwork-like pattern at low temperatures of 50 to 300° C. so aconductive nano-ink is obtained.

The inks (e.g., ink paste, inks, solutions, coatings) are especiallyadapted for use in or on top of transparent substrates. Theaforementioned ink is adapted for coating, covering, immersing, dipping,and/or entrapping on top or into either solid or semi-solid matrix, orby means of any other suitable technique on such as glass or any polymermatrix, including flexible, semi-flexible or rigid materials. In someembodiments, due to their significant transparency in the visiblewavelength range of about 400 nm to 700 nm, the aforementionedconductive inks are especially useful for screens, displays, liquidcrystalline displays, smart cards and/or for any technology usingink-jets printers or any other technology of printing electronic matter.

The coating techniques that we have used are screen-printing, manualapplicator and manual spreading. Other suitable techniques such as spincoating, spray coating, ink jet printing, offset printing and anysuitable technique can also be used. Any type of transparent andnon-transparent substrate can be coated for example glass,polycarbonate, polymer films and others.

Various ink/paste and coating systems have been found useful to producea transparent-conductive coating, which differ in the formulationconcept, and main ingredients leading to the conductivity andtransparency. The main ingredients are selected from metal nano powder;metal nano powder with metal colloids; metal nano powder with a metalreducible salt; and/or organic metal complexes and/or organo-metalcompounds which decompose to form conductive materials and all the abovein a self-organizing system.

These nano metal based coating systems can achieve light transmittancesof up to 95% (measured between 400 nm and 700 nm), low haze values, andresistances as low as 1 Ω/square.

The ink or pastes may be prepared according to the general proceduredescribed below. Care has to be taken to achieve a good dispersion ofthe conductive additives (metal nano powders, salts, colloids and otheradditives).

The substrate is coated with the ink or paste. The coating can beperformed using the different techniques previously described. Thetechnique is chosen to enable control of physical parameters such asthickness and printed geometries (to obtain the desired transmittanceand resistance). The sample may be heated, to obtain the desiredresistance, to between 50° C. to 300° C. for 0.5 to 2 hours.

In some embodiments, a substrate is coated with a solution or paste inwhich the nano metal powder is dispersed, and sintered at lowtemperatures of about 50° C. and preferably around 100° C. to 220° C. toproduce conductive layers characterized by resistances between 0.005Ω/square to 5 Ω/square. These resistance values are comparable toresistivity values of between 2.1·10⁻⁵ and 6.6·10⁻⁴ Ω·cm. The lowestresistivity obtained is about only 1.5 times higher than bulk silver asmeasured in our system. While commercially available and literaturecited technologies suggest that producing conductive layers requiressintering temperatures over 300° C. and usually near 900° C., thepresent invention discloses, in certain embodiments, a novel method ofsintering at temperatures lower than 250° C. and/or lower than the Tg ormelting point of flexible materials such as plastics and polymers.

In some embodiments, the term “sintering” refers to any process offorming objects from a metal powder by heating the powder at atemperature below its melting point. In the production of small metalobjects it is often not practical to cast them. Through chemical ormechanical procedures a fine powder of the metal can be produced. Whenthe powder is compacted into the desired shape and heated, i.e.,sintered, for up to three hours, the particles composing the powder jointogether to form a single solid object.

The nano metal particle-containing composition, in which the particlesare dispersed in a liquid carrier, is printed, preferably by ink jetprinting, onto the surface of a substrate. The resulting pattern canhave any shape, geometry, or form achievable using the particularprinting technique. A representative pattern is shown in FIG. 12.

FIGS. 13 and 14 illustrate representative printed patterns prior tosintering. The patterns, which are gray to dark gray in color, includedrop-like shapes attributable to the ink jet printing process.

Heat sintering such printed patterns, e.g., at temperatures on the orderof 150° C. to 200° C., yields a different result compared to chemicalsintering at much lower temperatures, e.g., on the order of 70° C. Thiscan be seen by comparing (a) FIGS. 15 and 16, which depict printedpatterns prior to sintering, (b) FIGS. 17 and 18, which illustrateprinted patterns heat sintered at 150° C. and (c) FIGS. 19 and 20, whichillustrate printed patterns chemically sintered at 70° C. It is clearthat during the low temperature chemical sintering process (“CSM”) theparticles grow substantially, forming bigger particles compared to theheat sintered material. This has been correlated by surface areameasurements performed on dry powder sintering (see the followingtable). “FAV” refers to formic acid vapor.

TABLE A Surface Area Change with Sintering Process. Surface Area PowderSintering method mt²/gr 414 L303 Before sintering 6.5 414 L303 60° C.,24 hr 5.3 414 L303 CSM - FAV, 60° C., 24 hr 1.4 473-G51 W025 Beforesintering 8.3 473-G51 W025 200° C., 0.5 hr 8.5 473-G51 W025 300° C., 0.5hr 6.9 473-G51 W025 CSM - FAV, 60° C., 24 hr 3.5

FIGS. 21 and 22 further illustrate the change in particle size thataccompanies exposure of metal nano particles, in the form of dry powder,to formic acid vapor at 70° C. The particles also experienced a decreasein surface area from an initial value of 10.3 mt²/g to 2.5 mt²/g.

FIGS. 15-20 further illustrate that both heat sintering at 150° C. andchemical sintering at 70° C. results in particles that substantiallyretain their original, non-uniform shapes. After heat sintering,however, the printed pattern retains its dark gray color, whereas afterchemical sintering the particles have a silver-like, yellowish color.

The invention will now be described further by way of the followingexamples.

EXAMPLES

Examples for formulations for each method are described below. These areonly representative examples and are described hereby to demonstrate thewide range of possibilities this invention covers, by which we can usethe special properties of nano metal powders. It is further acknowledgedthat the formulations of the hereto-described examples may similarly bemade with different binders, solvents, metal powders, additives,polymers, buffers, surfactants, dispersants and/or coupling agents.Nevertheless, according to the present invention, nano powder metalsand/or nano powder metal salts characterized by small particle size(D₉₀<0.1 μm) which are conductive are especially preferred.Concentrations can be adjusted to control the viscosity and theresistance and transparency of the coated substrate.

Example 1

Admixing a binder (ethyl cellulose; MW=100,000), 13% by weight, in anaromatic ester or aldehyde solvent (terpinol). Further homogenizing 25parts by weight of the obtained binder solution by a means of a high rpmhomogenizer the following materials: Silver nano-powder (D₉₀<0.1 μm),50% (w/w), a solvent (terpinol) 19% (w/w) and a coupling agent(isopropyl dioleic(dioctylphosphato)titanate, also known as thecommercially available NDZ-101 KRTTS (Kenrich Petrochemicals, Inc.,Bayonne, N.J.), 1% (w/w).

Example 2

Admixing metal nano powder (colloidal silver), 12% (w/w), a binder whichis an adhesion promoter (Polyvinyl Pyrrolidone (PVP)), 2.5% (w/w), anddeionized water, 32% (w/w). All components are intensively mixed by ameans of an ultrasonic energy and/or high rpm dispersing equipment.Separately, admixing metal nano powder (silver nano powder) (D₉₀<0.1μm), 14% (w/w), solvent (ethanol), 39.5% (w/w), by a means of a high rpmhomogenizer. Finally, admixing solution obtained in first step with thesolution obtained in the second step, so a homogenized solution isobtained.

Example 3

Admixing silver formate salt—1 part by weight, a dispersant(trioctylphoshine oxide) (TOPO), 2% (w/w) in a solvent (ethyl acetate),80% (w/w). Heating the solution to about 60° C. until all componentsdissolve. Admixing metal nano powder (silver powder) (D₉₀<0.1 μm), 17%(w/w) into the brown solution obtained in the first step, andsubsequently homogenizing the obtained mixture by a means of a high rpmhomogenizer.

Example 4

Dissolving silver nitrate, 4% (w/w) in a buffer (ammonia, 25% solution,9.5% (w/w)). Further admixing deionized water, 47% (w/w) and adispersant (poly-carboxylic ammonium salt, commercially available asTamol 1124 product from Rohm and Haas Co., Philadelphia, Pa.), 3.5%(w/w), solvent (ethanol), 24% (w/w) and metal nano powder (silverpowder) (D₉₀<0.1 μm), 12% (w/w) and subsequently homogenizing theobtained mixture by a means of a high rpm homogenizer.

The optical and resistance data for metal nano powder coatings and inksproduced in the hereto-defined examples are hereto provided:

TABLE 1 Optical and resistance data for nano metal powders coatings andinks Example Printing Transmit- Resistance No. Geometry tance %ohmsquare Haze % Remarks Blank 92 8 Glass slides 1 Screen 70.3 14 1.8 UVopacity 2 Continuous 46.3 80 8.6 UV opacity 3 Continuous 54.5 30 9.1 UVopacity 4 Continuous 55.6 500 3.3 UV opacity

Light transmittance measured between 400 nm and 700 nm. Heat treatmentwas provided at 280° C. for about one hour.

It is hence according to another embodiment of the present invention, topresent a new and simple method of producing transparent and conductivecoatings and inks on top or in glass and/or polymeric surfaces. Thisnovel method is based on spreading of a predetermined mixture definedabove on the surface to be coated. While an organic solvent isevaporated from the spread mixture, the self-assembled network-likepattern is developed in situ, i.e., on the surface. After drying iscompleted, the developed pattern is sintered at relatively lowtemperatures, e.g. at a temperature ranging from 50° C. to 150° C. forabout 2 to 30 minutes. The final resistance of the conductive layer liesin the range of about 1 to 1,000 Ohm/square, light transmittance is inthe range of about 50 to 95%, and the haze value is in the range ofabout 0.5 to 5%.

The said special mixture is a W/O type emulsion of water or watermiscible solvent (or a mixture of these solvents) in a suspension ofmetal fine particles in an organic solvent or mixture of two or moresolvents not miscible with water.

The mixture may also contain at least one emulsifying agent, binder orany mixture thereof. Hence, the dispersed phase is selected from thegroup comprising yet not limited to water, methanol, ethanol, ethyleneglycol, glycerol, dimethyl formamid, dimethyl acetamid, acetonitrile,dimethyl sulfoxide, N-methyl pyrrolidone and/or other water misciblesolvents.

The continuous phase may be selected from the group comprising yet notlimited to petroleum ether, hexanes, heptanes, toluene, benzene,dichloroethane, trichloroethylene, chloroform, dichloromethane,nitromethane, dibromomethane, cyclopentanone, cyclohexanone or anymixture thereof. Preferably, the solvent or solvents used in thiscontinuous phase are characterized by higher volatility than that of thedispersed phase.

Similarly, the emulsifying agents are selected from the group comprisingyet not limited to non ionic and ionic compounds, such as thecommercially available SPAN-20, SPAN-80, glyceryl monooleate, sodiumdodecylsulfate, or any combination thereof. Moreover, the binders areselected from, yet not limited to modified cellulose, such as ethylcellulose MW 100,000-200,000, modified urea, e.g., the commercialavailable BYK-410, BYK-411, BYK-420 produced by BYK-Chemie Ltd.

The metal fine particles and nano-powders are selected from, yet notlimited to silver, gold, platinum, palladium, nickel, cobalt, copper orany combination thereof, wherein said metal or mixture of metals ischaracterized by an average particle size smaller then 1 micron,preferably smaller than 0.5 micron, and most preferably smaller then 0.1micron. The small particle size provides the improved optical propertiesof the coatings. According to one specifically preferred embodiment, theaforementioned metal or mixture of metals is selected from preciousmetals due to their enhanced chemical stability and improved electricalconductivity.

TABLE 2 Basic mixture formulation of the nano-inks and nano-powdersobtained by the method as defined and described in the above mentionedexamples. Component Min. content, % Max. content, % Organic solvent ormixture 40 80 Binder 0 3 Emulsifying agent 0 4 Metal powder 2 10 Watermiscible solvent or mixture 15 55

The mixture may be prepared in the following way: Emulsifying agentand/or binder dissolved in organic solvent or mixture and metal powderadded. Metal powder dispersed in organic phase by ultrasonic treatment,high shear mixing, high speed mixing or any other way used forpreparation of suspensions and emulsions. After this water misciblesolvent or mixture added and W/O type emulsion prepared by ultrasonictreatment, high shear mixing, high speed mixing or any other way usedfor preparation emulsions.

The above-mentioned self-assembled network pattern can be developed ondifferent surfaces: glass, polymeric films and sheets (polyesters,polyamides, polycarbonates, polyethylene, polypropylene etc.). Thesurface to be coated may be both untreated and treated to change itssurface properties (e.g., corona treatment or coating by primer).Example of primers that may be used include 1-2% acetone or hexanesolutions of 3-Aminopropyl Triethoxy silane, Phenyl Trimethoxysilane,Glycidyl Trimethoxysilane, Tween-80, NeoalkoxyTri(dioctylpropylphosphato) titanate, etc.

Suitable coating techniques include simple spreading, spin coating,dipping, etc. Wet thickness of coating preferably ranges from 5 to 200microns.

It is also possible to develop the self-assembled network on glass andthen transfer the pattern to a polymer by printing on the polymer.

Example 5

Admixing a surfactant (SDS), 0.1 g in 40 g of water. Admixing then abinder (ethyl cellulose, MW=100,000), 1 g, in a solvent (toluene), 60 gand nano powder metal (silver nano-powder) (D₉₀<0.1 μm), 8 g. Then,homogenizing the obtained solution by a means of ultrasonic energyand/or high rpm dispersing equipment. Lastly, admixing 31 g of thesolution obtained by the first step with the solution obtained by thesecond step. This formulation was spread on a glass surface by a manualapplicator with a wet thickness of 30 microns. After drying, aself-assembled network pattern developed. Then this pattern was sinteredat 150° C. for 10 min. The resulting resistance of the layer was 16Ω/square. The layer also had a transparency of 55.3% and a haze of 5%.

Example 6

Admixing a metal nano powder (silver powder; maximal particle size islower than 0.12 micron), 4 g; a solvent (1,2-Dichloroethane), 30 g; abinder (BYK-410), 0.2 g; and homogenizing the solution by a means of anultra sound at 180 W power for 1.5 min. Then admixing distilled water,15 g and homogenizing the obtained emulsion by an ultra sound means at180 W power for 30 sec. This formulation was printed on a glass surfaceand provided a good developed network with big cells (above 40 μm up to70 μm and lines about 2-6 μm width). Resistance was 7 Ω/square,transparency 75%, and haze value 1.8%. Reference is made now to FIG. 1,presenting a view taken by a means of a light microscope showing thepattern on glass surface as obtained by the method as described inExample 6.

Example 7

Admixing metal nano powder (silver powder; maximal particle size islower than 0.12 micron), 4 g; a solvent (toluene), 30 g, a binder(BYK-410), 0.2 g, and homogenizing the solution by a means of an ultrasound at 180 W power for 1.5 min. Then admixing distilled water, 15 gand homogenizing the obtained emulsion by an ultra sound means at 180 Wpower for 30 sec. This formulation printed on a glass surface gives agood developed network with small cells (about 10 μm and lines about 1to 4 μm width). Resistance was 7 Ω/square, transparency was 65%, andhaze value 3.0% after sintering at 150° C. for 5 min. Reference is madenow to FIG. 2, presenting a view taken by a means of a light microscopeshowing the pattern on glass surface as obtained by the method asdescribed in Example 7.

Example 8

Admixing a binder (BYK-410), 0.06 g; a surfactant (SPAN-80), 0.03 g; asolvent (toluene), 16 g; a co-solvent (cyclopentanone) 0.8 g; nanopowder metal (silver powder; (maximal particle size is lower than 0.12micron), 0.8 g and homogenizing the obtained solution by an ultra soundmeans at 180 W power for 30 seconds. Then admixing distilled water, 9 mland homogenizing the obtained emulsion by an ultra sound means at 180 Wpower for 20 sec. This formulation was printed on glass pretreated with1% acetone solution of 3-aminopropyl triethoxy silane and gave a gooddeveloped network with cells of 40 μm to 70 μm and lines about 2 to 6 μmwidth. Resistance was 2 Ω/square, transparency was 72%, and haze valuewas 1.2% after sintering at 50° C. for 30 min in formic acid vapor. Thepattern was printed from glass to different polymeric films with almostno change in electrical and optical properties.

Example 9

Admixing metal nano powder (silver powder; maximal particle size islower than 0.12 micron), 4 g; a solvent (trichloroethylene), 30 g; abinder (BYK-410), 0.2 g; and homogenizing the obtained solution by anultra sound means at 180 W power for 1.5 min. Then admixing distilledwater, 15 ml and homogenizing the obtained emulsion by an ultra soundmeans at 180 W power for 30 sec. This formulation was printed on a glasssurface and gave a good developed network with small cells (about 10 to40 μm and lines of about 2 to 4 μm width). Resistance was 40 Ω/squareand transparency was 52% after sintering at 150° C. for 5 min. Thisformulation was also useful in the case when a high-speed stirrer(Premier-type) is used instead of the ultra sound sonication. In thiscase, the obtained network cells are bigger and the transparency isimproved. This formulation also has better performance while printed onpolymer films not dissolved by trichloroethylene, such as PET, PEN,polyethylene. Reference is made now to FIG. 3, presenting a view takenby a means of a light microscope showing the pattern on a glass surfaceas obtained by the method as described in Example 9.

Example 10

Admixing a binder (BYK-410), 0.06 g; a surfactant (SPAN-20), 0.03 g, asolvent (trichloroethylene), 24 g; a co-solvent (cyclohexanone), 0.8 g;metal nano powder (silver powder; maximal particle size is lower than0.12 micron), 1 g and homogenizing the obtained solution by an ultrasound means at 180 W power for 30 sec. Then admixing distilled water, 9ml and homogenizing the obtained emulsion by an ultra sound means at 180W power for 20 sec. This formulation was printed on glass pretreatedwith 1% acetone solution of 3-aminopropyl triethoxy silane and gave agood developed network with cells of 50 μm up to 100 μm, and lines about2 to 8 μm width. Resistance was 1.8 Ω/square, transparency 81.3%, hazevalue 2.6% after sintering at 50° C. for 30 min in formic acid vapor.Reference is made now to FIG. 4, presenting a view taken by a means of alight microscope showing the pattern on glass surface as obtained by themethod as described in example 10. The pattern printed on variouspolymeric films and did not change its electrical and optical propertiessignificantly. Reference is made now to FIG. 5, presenting a view takenby a means of a light microscope showing the pattern on polymeric filmas obtained by the method as described in Example 10.

Example 11

Admixing a binder (BYK-411), 0.06 g; a surfactant (SPAN-80), 0.2 g; asolvent (petroleum ether), 10 g; a co-solvent (cyclohexanone), 1 g;metal nano powder (silver powder; (maximal particle size is lower than0.12 micron), 1 g and homogenizing the obtained solution by an ultrasound means at 180 W power for 30 sec. Then admixing distillated water,7 ml and homogenizing the obtained emulsion by an ultra sound means at180 W power for 20 sec. This formulation was printed on polyimidepolymer pretreated with 1% acetone solution of phenyl trimethoxysilaneand gave a good developed network characterized by cells of above 20 μmup to 60 μm, and lines about 2 to 6 μm width. The resistance was 20Ω/square, transparency was 78%, and the haze value was 8% aftersintering at 150° C. for 5 min.

Example 12 Dry Nano Silver Metal Powders

Silver powders of different sizes, including nano size powders, wereproduced through the procedure described in U.S. Pat. No. 5,476,535,which is hereto provided as a reference. The powders are coated withorganic materials and de-agglomerated. The volume particle sizedistribution of these powders, measured in a Coulter Particle SizeAnalyzer LS 230, are presented in Table 3.

TABLE 3 Silver powders used in experiments Particle Size SampleDistribution Number D₅₀ μm D₅₀ μm 1 0.054 0.067 2 0.054 0.066 3 0.0520.063 4 0.246 2.851 5 3.22 8

The electrical resistances of these powders were measured as a functionof the sintering process, see Tables 4 and 5.

Reference is made now to FIG. 7, presenting the change of relativeresistance for silver nano powders coated with surfactant (1) and washedfrom surfactant (2). Reference is made now to FIG. 8, presenting therelative resistance dependence on temperature for the different particlesize silver powders—and measured bulk silver in our measuring system.

Samples 1, 2 and 3 are nano silver powders; samples 4 and 5 are coarsesilver powders with a particle size of over 2.5 μm (D₉₀). As can be seennano silver powder washed from its coating will give the sameperformance at even lower temperatures of about 100° C. in comparison toaround 220° C. for the coated powder and over 700° C. for coarse silverpowders.

TABLE 4 Electrical properties of silver powders Sample 2 (washed Sample1 (coated from surfactant) with surfactant) Temper- ResistanceResistivity Temperature Resistance Resistivity ature R, Ω ρ, Ω * cm ° C.R, Ω ρ, Ω * cm ° C. 1300 7.60 236 0.1065 1.24E−3 62 131 076 270 0.01661.94E−4 127

TABLE 5 Electrical properties of different particle size silver powdersat different sintering temperatures. T = 120° C. T = 220° C. SampleResistance Resistivity Resistance Resistivity Number R, Ω ρ, Ω * cm R, Ωρ, Ω * cm 2 0.0203 2.40E−4 0.0034 3.98E−5 4 0.1600 1.20E−3 0.08606.61E−4 5 0.4620 3.24E−3 0.4200 2.95E−3 Bulk Silver* 0.0040 1.95E−50.0045 2.14E−5 *Bulk silver measured under same conditions and set-up.

Example 13 Dry Nano Copper Metal Powders

Copper powders of different sizes, including nano sizes, including nanosize powders were produced through the procedure described in U.S. Pat.No. 5,476,535, which is hereto provided as a reference. The powders werecoated with organic materials and de-agglomerated. The volume particlesize distribution of these powders, measured in a Coulter Particle SizeAnalyzer LS 230, are presented in Table 6.

TABLE 6 Copper powders used in experiments Particle Size SampleDistribution Surface area Number D₅₀ μm Mean μm M²/g AS0873 0.073 0.18178.2 ASX0871 0.35 0.317 6.0 ASX13-1 3.4 3.4 1.3

The electrical resistances of these powders were measured as a functionof the sintering process. Reference is made hence to FIG. 9, presentingthe relative resistance dependence on temperature for the differentparticle size copper powders—and measured bulk copper in our measuringsystem.

Example 14 Nano Metal Powders in Formulations

The formulations are inks or pastes, which facilitate the printingand/or coating process, and were prepared according to the generalprocedures described below. Care has to be taken to achieve a gooddispersion of the conductive additives (metal nano powders, salts and/orcolloids).

Three ink/paste systems were tested. All three have been found toproduce a conductive coating at low sintering temperatures. The systemsdiffer in the formulation concept and main ingredients leading to theconductivity. The main ingredients of the systems are: 1) metal nanopowder, 2) metal nano powder with metal colloids, 3) metal nano powderwith a metal reducible salt.

Examples for formulations for each method are described below.Resistance results for these systems re presented in Table 7.

System 1 Formulation

Admixing a binder (ethyl cellulose), 13% (wt/wt) in a solvent(terpinol). Then, admixing a conductive nano powder metal (silver nanopowder) (D90<0.1 μm); 50 parts by weight; terpinol 20 parts by weight,and a coupling agent such as isopropyl dioleic(dioctylphosphato)titanate, also known as the commercially availableNDZ-101 KRTTS, 1 part by weight, to some 25 parts by weight of thesolution obtained above, by a means of a high rpm homogenizer.

System 2 Formulation

Intensively admixing colloidal silver, 12 parts by weight; a binderwhich is also an adhesion promoter (Polyvinyl Pyrrolidone (PVP)), 2.5parts by weight; water, 32 parts by weight by a means of an ultrasonicenergy and/or high rpm dispersing equipment. Then, admixing a conductivenano powder metal (silver nano powder) (D₉₀<0.1 μm), 14 parts by weight;solvent (ethanol), 39.5 parts by weight by a means of a high rpmhomogenizer. Finally combining the second mixture with the first mixturewhile mixing and stirring thoroughly.

System 3 Formulation

Admixing silver formate salt, 1 part by weight; a dispersant(trioctylphoshine oxide (TOPO)), 2 parts by weight; and a solvent (ethylacetate), 80 parts by weight at about 60° C. until all componentsdissolve. Then, admixing a conductive nano powder metal (silver nanopowder) (D₉₀<0.1 μm), 17 parts by weight to the obtained brown solutionby a means of a high rpm homogenizer.

TABLE 7 Resistance data for nano metal powders ink formulationsSintering Resistance Resistivity Temperature, System FormulationΩ/square Ωcm ° C. 1 P0010 0.7 2.84E−4 120 1 P0010 0.05 2.03E−5 300 3C116 2.8 6.72E−5 120 3 C116 1.17 2.93E−5 300 2 C121 0.255 3.09E−4 100Bulk Silver* 0.004 1.95E−5 120 Bulk Silver* 0.0045 2.14E−5 220 *Bulksilver measured under same conditions and set-up

In Examples 15-22, the nano metal powder was prepared as follows.

Silver nano powder was prepared by making a melt of 24.4% by weight ofsilver, 0.6% by weight copper and 75% aluminum (e.g., 243.8 gramssilver, 6.3 grams copper, and 750 grams aluminum) in a stirred graphitecrucible under air at a temperature of at least 661° C. The melt waspoured into a 14 mm thick mold made from steel. The molded ingot wasleft to cool at room temperature, and then annealed in an electricalfurnace at 400° C. for 2 hours. The annealed ingot was left to cool atroom temperature, and then rolled at room temperature in a rollingmachine (from 13 mm thickness to 1 mm thickness in 24 passes). Thesheets were cut and heat treated in an electrical furnace at 440° C. for4 hours. The heated sheets were quenched in water at room temperature.The sheets were then leached in an excess of a NaOH solution (25% byweight in deionized water—density 1.28 grams/ml at room temperature,1.92 kg NaOH solution per 0.1 kg alloy) at a starting temperature of 28°C. and while cooling to keep the temperature below 70° C. for 12 hours(leaching reactor without external agitation).

The NaOH solution was then decanted and a new portion of 25% NaOHsolution was added (40 gram per 0.1 kg starting alloy) and left for 2hours. The powder produced at this stage had a prime particle size below80 nm, as measured by XRD and SEM, and a surface area greater than 5mt2/gram. An ethanol solution was prepared by dissolving 15.66 gramsSpan 20 and 2.35 grams hexadecanol in 750 ml ethanol. 500 grams ofleached dry powder was added to the ethanolic solution and stirred for 2hours. The slurry was poured into a tray and the ethanol evaporated at atemperature below 45° C. The coated powder was then passed through a jetmill to get a de-agglomerated silver nano powder with particle size(D90) below 80 nm, as measured by laser diffraction.

The powder produced in the previous steps was further washed with hotethanol several times (between 3 and 5 times), and then dried in trayuntil all the ethanol evaporated at a temperature below 45° C.). Ade-agglomerated silver nano powder with particle size (D90) below 80 nm,as measured by laser diffraction, and organic coating of less than 1.2%by weight, as measured by TGA, was obtained.

Example 15

A dispersion of 20% by weight of silver nano powder (#473-G51) (preparedas described above), 1% Disperbyk® 190 (available from BYK-Chemie, WeselGermany), 0.027% BYK® 348 (also available from BYK-Chemie), 0.067% PVPK-15 (available from Fluka), 0.313% AMP (2-amino-2-methyl-propanol),15.76% NMP (1-methyl pyrrolidinone), and the balance water was preparedby mixing the additives with the solvents and water, then adding thesilver nano powder in portions while mixing at 4000 rpm with a highspeed Premier Mill Laboratory Dispersator Series 2000 Model 90 (PremierMill Corp. USA) having a 47 mm diameter dissolver shaft, until theminimum PSD was achieved (D100 77 nm). Typically, homogenization wasperformed for 10 min. The viscosity of the composition was determined tobet 15 cP using a Brookfield Viscometer. A surface tension of 47.5 mN/mwas measured using the Dunoy method. A conductive pattern was printedwith this dispersion using a Lexmark printer Z602, cartridge LexmarkBlack 17 and 16 in which the black ink had been replaced with thisdispersion. The dispersion was printed on HP photoquality papersemi-glossy (C6984A). Two passes were performed. The conductive patternwas exposed to formic acid vapor (formic acid/water solution, 85% byweight) at 70° C. for 30 minutes, after which its resistance wasmeasured and determined to be 0.23 Ω/square.

Example 16

The process of Example 15 was repeated. The resistance of the sample wasmeasured and determined to be 0.11 Ω/square. This is equivalent to aresistivity of 7 Ωcm (calculated by measuring the thickness of theprinted pattern with a surface profilometer).

Example 17

The process of Example 15 was repeated except that the dispersion wasprepared by diluting a dispersion having 60 wt. % silver nano powder. Aconductive pattern was printed with an applicator (draw bar) to form acoating having a wet thickness of 30 μm. The pattern was exposed toformic acid vapor (formic acid/water solution, 85% by weight) at 70° C.for 30 minutes, after which its resistance was measured and determinedto be 0.2 Ω/square. This is equivalent to a resistivity of 9 μΩcm(calculated by calculating the thickness of the printed pattern from theweight of the deposited material).

Example 18

The process of Example 17 was followed except that the printed pattern,after drying, was exposed to acetic acid vapors at 70° C. for 30minutes, after which its resistance was measured and determined to be 18Ω/square. This is equivalent to a resistivity of 500 μΩcm (calculated bycalculating the thickness of the printed pattern from the weight of thedeposited material).

Example 19

The process of Example 17 was followed except that the printed pattern,after drying, was exposed to formaldehyde vapors at 70° C. for 30minutes, after which its resistance was measured and determined to be 5Ω/square. This is equivalent to a resistivity of 200 Ωcm (calculated bycalculating the thickness of the printed pattern from the weight of thedeposited material).

Example 20

The process of Example 17 was followed except that the printed pattern,after drying, was dipped in acetic acid.

Example 21

The process of Example 17 was followed except that the printed pattern,after drying, was dipped in formaldehyde.

Example 22

The process of Example 17 was followed except that it was prepared with60 wt. % silver nano powder.

Example 23

Admixing a binder (BYK-410), 0.28 g; a surfactant (SPAN-80), 0.04 g, asolvent (trichloroethylene), 42 g; a co-solvent (toluene), 23 g; metalnano powder (silver powder; maximal particle size is lower than 0.12micron), 4 g and homogenizing the obtained solution by an ultra soundmeans at 180 W power for 1.5 min. Then admixing distilled water, 25 gand homogenizing the obtained emulsion by means of a high speed mixer at1000 rpm for 2 min. This formulation was spread on a glass surface andgave a good developed network Resistance was 30 Ω/square, transparencywas 70.3%, haze value was 2.6% after sintering at 150° C. for 5 min.

Example 24

Admixing a binder (BYK-410), 0.03 g; a surfactant (SPAN-80), 0.03 g, asolvent (trichloroethylene), 22.7 g; a co-solvent (cyclohexanone), 0.51g; metal nano powder (silver powder; maximal particle size is lower than0.12 micron), 3 g and homogenizing the obtained solution by an ultrasound means at 180 W power for 30 sec. Then adding a 0.005% watersolution of a substrate wetting additive (BYK-348), 16 g andhomogenizing the obtained emulsion by means of ultra sound at 180 Wpower for 20 sec. This formulation was spread by manual applicator on aglass surface pretreated with a 1% acetone solution of 3-aminopropyltriethoxy silane (wet thickness=30 microns) and gave a well-developedpattern. Resistance was 2.5 Ω/square, transparency was 89.1%, haze valuewas 2.6% after sintering at 50° C. for 15 min. in formic acid vapor.

Example 25

The formulation of Example 24 was spread on a 3-aminopropyl triethoxysilane-treated glass surface by manual applicator (wet thickness=60microns). Resistance was 0.8 Ω/square, transparency was 78.1%, hazevalue was 3.8% after sintering at 50° C. for 15 min. in formic acidvapor.

Example 26

Admixing a binder (BYK-410), 0.03 g; a surfactant (SPAN-80), 0.05 g, asolvent (toluene), 16.47 g; a co-solvent (cyclohexanone), 0.95 g; metalnano powder (silver powder; maximal particle size is lower than 0.12micron), 1.7 g and homogenizing the obtained solution by an ultra soundmeans at 180 W power for 30 sec. Then adding water, 16 g, andhomogenizing the obtained emulsion by means of ultra sound at 180 Wpower for 30 sec. This formulation was spread by manual applicator on aPPC polyester film (available from Jolybar Filmtechnics ProductsConverting (1987) Ltd., Netanya, Israel) (wet thickness=30 microns) andgave a well-developed pattern. Resistance was 7 Ω/square andtransparency was 85.1% after sintering at 50° C. for 15 min. in formicacid vapor.

Example 27

Admixing a binder (BYK-410), 0.05 g; a surfactant (SPAN-80), 0.05 g, asolvent (1,2-dichloroethane), 36.3 g; metal nano powder (silver powder;maximal particle size is lower than 0.12 micron), 1.7 g and homogenizingthe obtained solution by an ultra sound means at 180 W power for 30 sec.Then adding water, 8 g, and homogenizing the obtained emulsion by meansof ultra sound at 180 W power for 30 sec. This formulation was spread bymanual applicator on a PPC polyester film (available from JolybarFilmtechnics Products Converting (1987) Ltd., Netanya, Israel) (wetthickness=30 microns) and gave a well-developed pattern. Resistance was5 Ω/square and transparency was 82.8% after sintering at 50° C. for 15min. in formic acid vapor.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method of producing an article comprising: (a) providing acomposition comprising nano-particles dispersed in a liquid carriercomprising (a) water, a water-miscible organic solvent, or combinationthereof and (b) an organic solvent that evaporates more quickly than thewater, water-miscible organic solvent, or combination thereof, whereinthe combination of (a) the water, water-miscible organic solvent, orcombination thereof and (b) the organic solvent that evaporates morequickly than the water, water-miscible solvent, or combination thereof,together form an emulsion; (b) applying the composition to a surface ofa substrate; and (c) drying the composition to remove the liquidcarrier, whereupon the nano-particles self-assemble to form atransparent and electrically conductive coating in the form of anetwork-like pattern that includes interconnected traces definingrandomly shaped cells on the surface of the substrate, wherein thecoating is transparent to light at wavelengths in the range of 400 to700 nm.
 2. A method according to claim 1 comprising sintering thenano-particles at a temperature no greater than 300° C. followingapplication to the surface of the substrate.
 3. A method according toclaim 1 comprising sintering the nano-particles at a temperature nogreater than 150° C. following application to the surface of thesubstrate.
 4. A method according to claim 1 comprising sintering thenano-particles at a temperature no greater than 50° C. followingapplication to the surface of the substrate.
 5. A method according toclaim 1 comprising sintering the nano-particles at a temperature between50 and 300° C. following application to the surface of the substrate. 6.A method according to claim 1 comprising sintering the nano-particles ata temperature between 50 and 150° C. following application to thesurface of the substrate.
 7. A method according to claim 1 comprisingexposing the nano-particles to a chemical reagent following applicationto the surface of the substrate to reduce the electrical resistance ofthe network.
 8. A method according to claim 7 wherein the chemicalreagent is in the form of a vapor.
 9. A method according to claim 7wherein the chemical reagent is in the form of a liquid.
 10. A methodaccording to claim 7 wherein the chemical reagent comprises an organicacid.
 11. A method according to claim 10 wherein the organic acidcomprises formic acid.
 12. A method according to claim 10 wherein theorganic acid comprises acetic acid.
 13. A method according to claim 7wherein the chemical reagent comprises an organic aldehyde.
 14. A methodaccording to claim 13 wherein the organic aldehyde comprisesformaldehyde.
 15. A method according to claim 7 comprising exposing thenano-particles to the chemical reagent at ambient temperature.
 16. Amethod according to claim 1 comprising sintering the nano-particles at atemperature no greater than 300° C. in the presence of an organic acidselected from the group consisting of formic acid, acetic acid, andcombinations thereof following application to the substrate.
 17. Amethod according to claim 1 comprising sintering the nano-particles at atemperature no greater than 150° C. in the presence of an organic acidselected from the group consisting of formic acid, acetic acid, andcombinations thereof following application to the substrate.
 18. Amethod according to claim 1 comprising sintering the nano-particles at atemperature no greater than 50° C. in the presence of an organic acidselected from the group consisting of formic acid, acetic acid, andcombinations thereof following application to the substrate.
 19. Amethod according to claim 1 comprising sintering the nano-particles at atemperature between 50 and 300° C. in the presence of formic acidfollowing application to the substrate.
 20. A method according to claim1 wherein the substrate is selected from the group consisting of glasssubstrates, rigid polymer substrates, flexible polymer substrates, andcombinations thereof.
 21. A method according to claim 1 wherein thesubstrate is selected from the group consisting of metal substrates,ceramic substrates, and combinations thereof.
 22. A method according toclaim 1 wherein the substrate is a paper substrate.
 23. A methodaccording to claim 1 wherein the substrate comprises a polymer substratehaving a glass transition temperature no greater than 150° C.
 24. Amethod according to claim 1 comprising applying the composition to thesurface of the substrate according to a predetermined pattern.
 25. Amethod according to claim 1 wherein the nano-particles comprise a metalelement.
 26. A method according to claim 1 wherein the nano-particlescomprise a metal oxide.
 27. A method according to claim 1 wherein thenano-particles include a metal element selected from the groupconsisting of silver, gold, platinum, palladium, nickel, cobalt, copper,and combinations thereof.
 28. A method according to claim 1 wherein thenano-particles are prepared according to a process comprising: (a)forming an alloy comprising an auxiliary metal and a metal; and (b)treating the alloy with a leaching agent to remove the auxiliary metal.29. A method according to claim 28 wherein the auxiliary metal comprisesaluminum.
 30. A method according to claim 1 wherein the nano-particleshave a D90 value of less than 0.1 μm.
 31. A method according to claim 1wherein the water-miscible organic solvent is selected from the groupconsisting of methanol, ethanol, ethylene glycol, glycerol,dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide,N-methylpyrrolidone, and combinations thereof.
 32. A method according toclaim 1 wherein the organic solvent that evaporates more quickly thanwater, the water-miscible solvent, or combination thereof is selectedfrom the group consisting of petroleum ether, hexane, heptane, toluene,benzene, dichloroethane, trichloroethylene, chloroform, dichloromethane,nitromethane, dibromomethane, cyclopentanone, cyclohexanone, andcombinations thereof.
 33. A method according to claim 1 wherein thecomposition further comprises an agent selected from the groupconsisting of polymers, binders, surfactants, dispersants, couplingagents, and combinations thereof.
 34. A method according to claim 1wherein the network has a percent light transmission of 10% to 90% at awavelength of 400 nm to 700 nm.
 35. A method according to claim 1wherein the network has a percent light transmission of 30% to 90% at awavelength of 400 nm to 700 nm.
 36. A method according to claim 1wherein the network has a haze value of 0.1% to 10%.
 37. A methodaccording to claim 1 wherein the network has a haze value of 0.50% to50%.
 38. A method according to claim 1 wherein the network has anelectrical resistance of 0.1 Ω/square to 1,000 Ω/square.
 39. A methodaccording to claim 1 wherein the network has an electrical resistance of0.1 Ω/square to 10,000 Ω/square.
 40. A method according to claim 1wherein the network has an electrical resistance of 0.1 Ω/square to10,000 Ω/square and has a percent transmission of 10% to 90% at awavelength of 400 nm to 700 nm.
 41. A method according to claim 1further comprising transferring the network to a second substrate.
 42. Amethod according to claim 1 comprising applying the composition to thesurface of the substrate by ink jet printing.
 43. A method of producingan article comprising: (A) providing an emulsion comprisingnano-particles dispersed in a liquid carrier that includes (a) water, awater-miscible organic solvent, or combination thereof and (b) anorganic solvent that evaporates more quickly than the water,water-miscible solvent, or combination thereof, wherein the combinationof (a) the water, water-miscible organic solvent, or combination thereofand (b) the organic solvent that evaporates more quickly than the water,water-miscible solvent, or combination thereof, together form theemulsion, wherein the nano-particles have a D90 value of less than 0.1μm and are prepared according to a process comprising: (i) forming analloy comprising aluminum and a metal; and (ii) treating the alloy witha leaching agent to remove the aluminum; (B) applying the composition toa surface of a substrate; (C) drying the composition to remove theliquid carrier, whereupon the nano-particles self-assemble to form anarticle comprising the nano-particles in the form of an electricallyconductive network on the surface of the substrate; and (D) sinteringthe nano-particles at a temperature no greater than 50° C. in thepresence of an organic acid following application to the surface of thesubstrate, wherein the network is transparent to light at wavelengths inthe range of 400 to 700 nm and has an electrical resistance of 0.1Ω/square to 1,000 Ω/square.